On the Absence of Broad Mg II Emission Line Variability in NGC 3516 during 1996

During the 1996 Hubble Space Telescope/Faint Object Spectrograph monitoring campaign of the Seyfert 1 galaxy NGC 3516, the UV continuum showed a factor of 5 variation; correspondingly, the strongest broad UV emission lines (e.g., Lyα, C IV) displayed somewhat weaker, although still significant, variations (a factor of 2). In contrast, no variation was detected in the strong broad Mg II + UV Fe II emission line complex. While historically the flux in the broad Mg II emission line has varied (a factor of 2 within a few years), the profile shape has not been observed to change over the past decade. In contrast, the high ionization lines (HILs) show both emission-line flux and profile changes on relatively short timescales, which appear to be correlated with changes in the continuum level. Significantly, on 1996 February 21, the profile shapes of the HILs (e.g., Lyα, C IV) and low ionization lines (e.g., Mg II) were identical, at which time the continuum level was at its highest. These results impose stringent constraints upon viable kinematic models of the broad-line region in NGC 3516, some of which we discuss in this Letter.